1. Field of the Invention
This invention pertains to spring units and in particular to a unitized spring structure of coil springs attached to a wire grid.
2. Description of the Prior Art
Prior spring units have generally been divided into essentially two categories. A first category includes a wire top grid having intricate wire bends forming locking members which are used to positively secure and enclose or capture the top coil of a spring such as illustrated in U.S. Pat. No. 3,577,574. The intricate bend arrangement generally requires the use of special assembly tools in attaching the top coil to the wire grid.
A second type of spring unit is illustrated in U.S. Pat. No. 3,864,765 which discloses a bedspring structure having a top grid with transversely extending wire pairs attached above longitudinally extending wires. The top coil of each bedspring is then placed above the lower wire but below the upper wire of the grid work and thus positioned in the grid. The top wires, that is the transversely extending wires of the grid, include indentations or offsets that nest into the top coil and frictionally retain each spring in position.
In addition to the problem mentioned above in connection with the first category, both categories of spring units present other problems by requiring complicated bends and indentations in the top grid wires which induce into the thin wires critical stresses that can cause premature failure. Secondly, because the grid wires have been deformed, the orientation, spacing and alignment of the wires, prior to welding and insertion of the coil spring, requires precise, time consuming positioning in both transverse and longitudinal directions.